A hydraulic drive system for a hydraulic working machine is provided with an engine, a variable displacement hydraulic pump drivable by power transmitted from the engine, a hydraulic actuator drivable by hydraulic oil delivered from the variable displacement hydraulic pump, an actuator control valve interposed between the variable displacement hydraulic pump and the hydraulic actuator and switchable between a feed state, in which hydraulic oil is fed to the hydraulic actuator, and a non-feed state, in which the hydraulic oil is not fed to the hydraulic actuator but is returned to a hydraulic oil reservoir, and an exhaust gas purification system for trapping in a filter particulate matter in exhaust gas as produced by incomplete combustion in the engine.
The exhaust gas purification system is arranged in an exhaust pipe through which exhaust gas from the engine is guided to an exterior of the hydraulic working machine. The particulate matter trapped in the filter of this exhaust gas purification system is combusted by heat of the exhaust gas, and therefore, is eliminated from the filter.
The hydraulic working machine is constructed such that in its non-operation state, in other words, in the above-described non-feed state, an engine output is reduced, for the purpose of energy saving, to a level needed for the variable displacement hydraulic pump to deliver pressure oil at a lowest delivery pressure and a smallest delivery rate required for cooling and lubricating a hydraulic circuit.
When the engine output drops, the temperature of exhaust gas also drops. As a consequence, the combustion of particulate matter by the heat of the exhaust gas is rendered difficult to occur, and accordingly, the filter of the exhaust gas purification system becomes prone to clogging. To avoid clogging of the filter, the conventional hydraulic drive system for the hydraulic working machine is constructed such that, when clogging of the filter is detected, the delivery pressure and delivery rate of the variable displacement hydraulic pump are increased to make greater a load to be applied to the engine and the temperature of exhaust gas is allowed to rise to a temperature needed for the combustion of particulate matter. A means for increasing the delivery pressure is a selector valve that can open or close a line through which delivery oil of the variable displacement hydraulic pump is guided from the variable displacement hydraulic pump to the hydraulic oil reservoir. The hydraulic drive system is constructed that in the non-feed state, the delivery pressure is increased by controlling the selector valve (see, for example, Patent Document 1).